Compensated oscillator system



Oct. 1l, 1949K. P C, GARDNER 2,484,562

COMPENSATED OSCILLATOR SYSTEM Filed Dec. 4, 1945 2 Sheets-Sheet 1 Figi. 2

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bym o M H i s Attoney.

Oct. 1l, 1949. P Q GARDNER 2,484,562

COMPENSATED OSCILLATOR SYSTEM Filed DSC. 4, 1945 2 Sheets-Sheet 2 paul CGardV eT;

bym A577401 H is Attorrwey Patented ct. 1l, 1949 COMPENSATED OSCILLATOR SYSTEM Paul C. Gardiner, Scotia, N. Y., assignor to General Electric Company, a corporation of New York Application December 4, 1945, Serial No. 632,729

(Cl. Z50-36) 9 Claims. 1

My invention relates to vacuum tube oscillators and more particularly to means for stabilizing .the performance of such oscillators under varying conditions of power supply voltage.

One of the 1principal methods of generating high frequency electric currents, particularly in the ultra-high and micro-wave frequency reg-ions, is `by .the use of va ,self-excited type oscillator having a cavity resonator, coaxial transmission line, `or other high Q resonant circuit. These oscillators have a high degree of reliability, can be easily and inexpensively constructed, and are Small in size. In addition, they may be readily constructed to operate to extremely high frequencies where other oscillator types have not been .found practical.

' Qne requirement of a satisfactory high frequency oscillator `is that a high degree of fr-eq-uency stability be achieved inasmuch as frequency disturbances considerably influence the performance of the radio, radar, .or other equipments .connected thereto. One of the factors influencing the frequency 4of operation of a selfexcited .Oscillator ,is the voltage applied to the .filament ,and plate circuits .of the tubes since variations in these supply voltages alter the effective tube characteristics and thereby change the frequency vof operation. In Yorder to avoid these'distu-rbances it vhas been common practice to use regulated power supplies for supplying these voltages, units which add substantially to the weight vand space requirements of the oscillator .and increase y:the expense thereof.

It :is an object yof lmy invention to .provide improved means .of stabilizing the frequency `of self- .excited oscillators.

An additional .object of my invention is t .stabilize the performance of a self-excited oscillator without r-egulating both filament and plate yvoltage supplies thereof. f It is a further object of my invention to sta- I,lsilize the yperformance of an oscillator in a manner whereby standard, low cost, circuit components may be used and a maximum .degree of simplicity and r-eliability obtained.

A further additional object of my invention is to stabilize the kperfor-mance of a self-excited oscillator in a v.manner which does not require .control of low voltage, high current, filament r.power supplies. f

Still .another object of .my invention is to sta- `hilize the performance of an oscillator even though power supply disturbances which would otherwise vary .the oscillating frequency take place at arapid rate.

Briefly, my invention resides in preventing frequency disturbances associated with power supply voltage changes in self-excited oscillators by providing a compensating change in some other power supply potential. That is, if a particular change in lam-ent supply voltage produces a frequency disturbance of, say, 1 kc., I change the plate supply voltage an amount suiiicient to produce a 1 kc. frequency change in the opposite direction, thereby compensating for the initial `change in filament supply voltage and achieving a stable voscillating frequency.

Further, I stabilize the performance of a self.- excited oscillator under rapidly changing filament supply voltage conditions by the use of a regulator which produces a plate voltage change in amount sufiicient to maintain constant frequency but having a time delay equivalent to the thermal time .delay inherent in the tube construction. That is, if the time interval between a filament voltage change and the corresponding changes in interelectrode capacitance, and other tube characteristics may be represented by an equiv.-

alent time constant of four seconds, I apply a plate circuit voltage controller having atime constant of four seconds, thereby causing the plate voltage change accurately to follow variations in tube characteristics.

The novel features which I believe to be characteristic of my invention are set forth with particularity inthe appended claims. My invention itself, however, both as to its organization and method of operation together with further objects and advantages thereof may best be understood vby ,reference to the following descrip,- tion taken in connection with the accompanying ,drawings in which Fig. 1 is a cross section View and partial schematic diagram of a stabilized high frequency .system embodying the invention and comprising a conventional microwave oscillator utilizing a lighthouse tube in combination with a rectifying and voltage regulating circuit; Fig. 2 shows the equivalent circuit of the oscillator portion of Fig. 1; Fig. 3 shows the relationship between the frequency of oscillation and the cathode heating current ,for an oscillator such as that of Fig. 1 when operating under typical conditions; Fig. 4 shows the relationship between t'hefrequency ,of oscillation and the plate voltage for an oscillator such as that of Fig. 1 when operating under typical conditions; Fig. 5 shows the relationship between cathode heating current and plate voltage for constant operating frequency for an oscillator such as that of Fig. 1-;

. iFig. 6 is a detailed schematic diagram of the the frequency of operation of the oscillator.

3 rectifying and voltage regulating circuit illustrated in block form in Fig. 1, whereby oscillator operation in accordance with the principles of my invention may be achieved; and Fig. '7 is a group of curves further illustrating a mode of operation of my invention.

Referring now to Fig. 1, there is illustrated ya typical microwave oscillator structure I2 in which a coaxial transmission line I is connected across grid t and cathode 5 of lighthouse tube 8 and a coaxial transmission line 2 is connected acrossv grid i and anode 3 of the same tube. Direct plate supply voltage is impressed upon a pair of terminals I, connected to anode 3 and to transmission line 2, respectively, from a rectifying and voltage regulating circuit 30. The circuit 30 is supplied from an alternating current supply source, not shown but indicated conventionally by terminals I5. The heater 9 may be supplied with cathode heating current from the same fsupply source through lament transformer 3| and a pair of terminals 6 connected directly to heater 9.

While the oscillator shown in Fig. 1 comprises a pair of coaxial transmission lines, its per-formance may be represented by an equivalent circuit comprising a modified tuned plate-tuned grid oscillator having a grounded grid. This equivalent circuit is shown in Fig. 2. It consists of 'parallel resonant circuit I0 representing coaxial transmission line I and parallel resonant circuit I i representing coaxial transmission line 2. The

'grid-anode capacity of tube 8 is represented tube characteristics of interelectrode capacity Yand equivalent space -path resistance. These characteristics alter the circuit impedances and therefore the natural frequency of oscillation of the system. Any external disturbances such as changes in heater voltage and current, plate voltage, etc., which alter the values of these tube characteristics, cause a corresponding 4change in If the heater current If applied to the tube is changed, for instance due to fluctuations in 4heater supply voltage, the temperature of the cathode, and consequently the physical size thereof, is altered from the normal condition.

'This changes both the tube grid-cathode capacitance and Ithe effective space path resistance, thereby altering the frequency of oscillation. .The magnitude and direction of this frequency change with variation in If is shown in Fig. 3 for the case of an oscillator such as .that of Fig. 1 and having all other operating conditions held constant.

Y Fig. 4 shows the change in `frequency of an oscillator such as that of Fig. 1 for various plate voltages Eb, all other circuit values being held constant. In this case, the plate Voltage varia- Ations alter the eiective space path resistance becompensating for the Variation due to the other.

These values may be plotted on a curve which I prefer to designate the constant frequency curve of the oscillator. This curve, for the typical oscillator similar to that shown in Fig. 1, is shown in Fig. 5.

While the above-discussed frequency changes associated with variations in heater current and plate voltage are considered only with reference to the self-excited lighthouse tube oscillator, a similar eiect takes place in other types of oscillators as well. A detailed discussion of observed disturbances on other self-excited oscillators at medium radio frequencies is given by Y. Kusonose and S. Ishikawa, in Frequency Stabilization of Radio Transmitters, Proc. I. R. E., vol. 20, p. 310, February 1932. As described in this paper, Ithe frequency Variations with other oscillators are similar to those shown in Figs. 3 and 4 herein.

One method of achieving oscillator operation along the constant frequency curve is to control the plate supply voltage in relation to the cathode heating Voltage. If the same source is used for both of these voltages, as shown in Fig. 1, this can be achieved by incorporating a voltage regulator in circuit 3U which varies .the applied plate voltage in response to the source voltage, the plate voltage following the curve of Fig. 5 in response to source voltage changes. Alternatively, the cathode heating current can be controlled to follow plate supply voltage changes, .the cathode heating current varying in accordance with Fig. 5 as plate supply voltage changes takes place. In this case, the Voltage regulator would of course be included in the cathode heater connections to terminals 6.

In general, I prefer to regulate plate supply Voltage in accordance with cathode heating voltage changes for the reason that low current, small size regulating equipment may be used and no unavoidable time delays exist in the regulating circuit.

Fig. 6 shows one circuit arrangement of the recrtifying and regulating circuit 30 whereby oscillator 'operation in accordance with my invention may be achieved. This circuit is described in detail in my Patent 2,247,082, the discussion of which is incorporated herein by reference.

`Brieiiy, the regulating circuit comprises tubes I6 and I1, together with gas discharge tube I8. The grid potential at tube II is determined both by the output voltage of iilter I9, acting through potentiometer 2i] and variable resistance 2|, and by the output Voltage 'of the plate voltage supply system acting through potentiometer 22 and variable resistance 23. The grid potential lof tube I7. as compared to relatively constant voltage drop of gasdischarge tube I8, determines the grid bias vof tube I1 and the flow of space current therethrough, thereby establishing the voltage drop in resistance 25 and the grid voltage of tube I6. Inasmuch as the grid potential of tube I6 determines the voltage drop in the space path of this tube, the output voltage -across terminals 'I is established by this grid potential. By proper choice of tubes I6, II and I8, together with `the values of resistances 20, 2|, 22 and 23, the output Voltage across terminals "I may be made to 4have any desired variation with respect to the value of output voltage from filter I9. As this voltage is proportional to power supply voltage at terminals I5, :and the heater voltage across terminals t is likewise proportional to lthe power supply voltage at terminals I5, .the plate supply voltage appearing across terminals 'I varies in :accordance with the filament current. By properly choosing the circuit constants, this variation `may be made to follow a curve such as that shown heater*supply voltage, but are delayed an amount determined by the thermal time constant and 'emission characteristics of the cathode. That is, if a sudden change in heater current to a value giving a particular frequency change takes place, the :actual frequency change willnot occur until corne time after the constant change. This is shown inthe curves of Fig. 7. Fig. 7 (a) shows fa sudden change in cathode heating current if; Whereas Fig. 7 (b) shows the resulting variation in Acathode temperature T. It will be observed that there is a time delay t1 bef-ore the temperature reaches a new steady state value. Therefore, the frequency, as shown in Fig. '7 (c) has a corresponding time delay t1 in `reaching a new steady state condition equal to that of Fig. 7 (b) 'In order to cause the `plate supply voltage to follow 'the curve of Fig. '5 at all times after a sudden heater current change such as that shown in Fig. '7, it is necessary to delay the plate voltage change in accordance with the actual variation of the -oscillator frequency. Hence, the plate supply voltage change should not suddenly takeplace but should follow `a delayed Curve such as is shown in Fig. '7 (c) This isshoWn in Fig. 7 (d) where theplate supply voltages required to compensate for the frequency change of Fig. 7 (c) areplotted.

4Automatic time delay :may be achieved in the voltage regulating circuit of Fig. 6, for instance, by "the use of a condenser 24 connected to the ygrid of tube l1. Since this condenser opposes any 'sudden change inthe voltage across its terminals, `the voltage regulating action appearing at terminals 26 is delayed. By proper choice of the capacity of condenser 24 in relation to the value of rcsistances 2l and 22, the time constant of the complete voltage regulating circuit may be made to correspond with the effective time constant of the cathode of the oscillator tube, thereby causing the plate supply voltage to follow the curve of Fig. at all times.

For purposes Iof explanation I have described operation of an oscillator along the constant frequency curve of Fig. 5. Most of the `benefit of my invention can be achieved, however, by operating close to this line rather than attempting exactly to match it. Within reasonable limits, f-or instance, the curves of Figs. 3 and 4 can be yapproximated by straight lines, thereby making the approximate curve corresponding to Fig. 5 1a straight line also. If operation along this straight line is achieved, frequency errors will be second order effects and `of little importance for most purposes. It is, therefore, only necessary that the anode and cathode heating voltages substantially follow the constant frequency curve.

Although I have described above and regard as most practical the regulation of plate supply voltage in accordance with heater current, it will be apparent to those skilled in the art that stabilized oscillator control may be achieved by the reverse process. In this case a'controller responsive to plate voltage and adapted to vary heater current in substantial accordance with Fig. 5 is provided, thereby maintaining substantially constant frequency operation.

While I have shown and described my invention as applied to a particular system of connec- 'tions and as embodying various devices diagrammat-ically shown, it `will be obvious to those skilled in the art that changes and modifications may be made without departing from my invention, *and I, therefore, aim in the appended Claims to cover all such changes and modifications as fall within the true spirit and scope of my invention- What I claim as new and desire to secure by Letters Patent of the United .States is:

yl.. In combination, an electron discharge voscillator havingan anode, and 1a heated cathode, means to supply voltage to said cathode to heat said cathode for operation, means to supply operating voltage between said `anode and said cathode, said oscillator producing oscillations which vary in frequency in accordance with both 'of -said voltages, and means to vary one of said voltages in response to the other of said voltages to maintain the frequency of oscillation substantially constant, said last mentioned means having a time delay circuit delaying the response thereof by a time interval substantially equal to the effective thermionic time constant of said oscillator.

2. In combination, an electron discharge oscillator having :an anode, and a heated cathode, means to :apply operating voltage between said anode and said cathode, means to supply voltage to said cathode to heat said cathode for operation, said oscillator producing oscillations varying in frequency in accordance with both of said voltages, and means to vary said cathode heating voltage in accordance with the value of said anode volt-age suiciently to maintain the frequency of oscillations produced by said loscillator substantially constant, said last mentioned means .including a time delay circuit delaying the response thereof by la time interval substantially equal to the effective thermionic time constant of said oscillator.

3. In combination, an electron discharge oscillator having an anode, and a heated cathode, means to supply voltage to heat said cathode for operation, means to supply operating voltage between said anode and said cathode, said oscillator producing oscillations varying in frequency in accordance with both of said voltages, and means to vary said anode voltage in response to the value of said cathode heating voltage in a sense and magnitude sufficiently to maintain the frequency of oscillations produced by said oscillator substantially constant, said last mentioned means including means for delaying the response thereof by a time interval substantially equal to the effective thermionic time constant of said cathode.

4. The method of operating an oscillator having an electron discharge device with an anode and a heated cathode and in which frequency varies in accordance with both cathode heating voltage and the voltage between the anode and the cathode characterized by the step of causing a delayed change of one of said voltages in response to the other of said voltages, the voltage change being in direction and amount sufficient to maintain the oscillating frequency substantially constant.

5. The method of operating an oscillator having an electron discharge device with an anode and a heated cathode and in which frequency varies in accordance with both cathode heating voltage and the voltage between the anode and the cathode characterized by the step of causing a delayed change of cathode heating voltage in response to the value of anode voltage, the cathode heating voltage change being in direction and amount suicient to maintain the oscillating frequency substantially constant.

6. In combination, an electron discharge oscillator having an anode and a heated cathode, means to supply voltage to said cathode to heat said cathode for operation, means to supply operating voltage between said anode and said cathode, said oscillator producing oscillations varying in frequency in accordance with both of said voltages, a single source supplying energizing voltage for said rst mentioned means and said last mentioned means, and means to vary said anode voltage in accordance With the voltage value of said source, said variations being in amount and direction to maintain substantially constant the frequency of oscillation and having a time delay substantially equal to the eifective thermionic time constant of said cathode.

7. In combination, an electron discharge oscillator having an anode and a heated cathode, means to supply voltage to said cathode to heat said cathode for operation, means to supply operating voltage between said anode and said cathode, said oscillator producing oscillations Varying in frequency in accordance with both of said voltages, means to vary the anode voltage in response to the value of cathode heating voltage, said variation being substantially along the constant frequency curve of said oscillator, said last mentioned means including a time delay circuit delaying the response thereof by a time constant substantially equal to the effective thermionic time constant of said cathode.

8. The method of operating an oscillator wherein the operating frequency is dependent on the value of cathode heating voltage and the value of anode voltage characterized by the step of causing a delayed change of anode voltage in response to cathode heating voltage disturbances. the anode voltage change being in direction and amount to maintain the oscillator frequency substantially constant and having a time delay substantially equal to the effective thermionic time constant of the cathode.

9. In combination, a source of alternating electromotive force, a rectifier, an electron discharge device oscillator having at least an anode and a heated cathode, means including a series connected electron discharge device having at least a cathode, grid, and anode to apply output voltage from said rectifier to the anode-cathode space path of the electron discharge device of said oscillator, means to supply heating voltage from said source to the cathode of the electron discharge device of said oscillator, said oscillator producing oscillations of magnitude dependent on the voltage applied to said anode-cathode space path and to said cathode, and means to control the impedance of said series connected device in accordance with both the output voltage of said rectier and the anode-cathode voltage at said oscillator to maintain substantially constant the frequency thereof.

PAUL C. GARDINER.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 1,813,488 Field July 7, 1931 1,943,302 Dow Jan. 16, 1934 2,162,520 Whitaker June 13, 1939 2,230,216 Boers Jan. 28, 1941 2,247,082 Gardiner June 24, 1941 2,281,205 Schock Apr. 28, 1942 2,361,745 Cawein Oct. 3l, 1944 

